Raw Water Pump vs Fresh Water Pump

[psneeld]

Originally Posted by delmarrey

I use to rebuild centrifugal pumps when in the Navy in the 60's. The first thing that would go were the wear rings between the impeller and the case (lower and lower flow). Next would be the blades on the impeller, and then the shafts. And in a couple cases I had to fill the eaten away interior of the pump case, from cavitation, with a Belzona filler then paint on the Belzona ceramic coating.

I find it hard to believe that one could run a centrifugal pump dry for very long w/o damaging the seals or packing/shafts. As well, if one were to run these with the valve closed or the inlet restricted it would get hot and start to cavitate, ruining the impeller and seals.

So answer this;
Why is it that ALL manufactures of marine pleasure boat engines us the rubber impeller pumps?

Are there not small enough centrifugal pumps to do the job?

BTW Centrifugal pumps also have to be run at a constant speed, not variable like on the belt/gear driven rubber pumps. That means one needs a separate motor (electric) to run the pump. (no battery/ no pump)

So when the engine is at idle a centrifugal pump may just pump in too much water cooling the motor too much. And at full RPM's, it may not pump enough especially if it's getting worn wear rings.

Rubber impeller pumps on marine motors have prevailed for longer then I I've been around for good reason. And I'm sure in the past 50 years someone has had a brain fart thinking the same thing as the OP above.

Whats been proven, is proven and that's that!

Effect of Internal Clearances on NPSHR | Centrifugal Pumps

http://www.maintenanceworld.com/Arti...fugalpumps.pdf

Think outside the box and you'll have all your questions answered....the problem with experience at one level and trying to apply it to $50 bilge/washdown pumps...well let's just say you are off base.

Ever know someone that's left their bilge pump on for a weekend like the other poster mentioned....ruined? Not.....

[psneeld]

Originally Posted by emeraldsea

I skimmed through and don't see any mention of the fact the most pleasure boats send the exhaust out with the raw water, which acts as a muffler. Its crucial that the water pump">raw water pump turn only when the engine turns, as its the exhaust pressure that keeps the water from getting into the engine. Water in engine=(

Should be a vented loop to protect against that...but you are correct that some certainly do like mine.

Just put the pump on a solenoid that only runs the pump when the engine is running.

As I said...it needs to be engineered as it is less idiot proof than an EXPENSIVE engine mounted pump.

[emeraldsea]

The vented loop stops water from siphoning in when not running, as usually the engine is below the water line. With no exhaust pressure, it fills the engine. A pump will push water right past the vented loop, same as it does when running.

[psneeld]

Originally Posted by emeraldsea

The vented loop stops water from siphoning in when not running, as usually the engine is below the water line. With no exhaust pressure, it fills the engine. A pump will push water right past the vented loop, same as it does when running.

Was thinking like in my old system that gravity would be enough but in cases that it's not...still easy to overcome.

[Florida Mike]

Back in the early 50s, the oyster boats in my hometown used whatever engines they could get out of a junkyard they used a three speed manual transmission with a short piece of driveshaft that hooked to the regular shaft with a thrust bearing. They didn’t bother with the clutch they just shoved it in and out of gear. On the front of the engine they would hook up a belt driven washing machine pump to pump seawater. No watercooled manifolds just a straight pipe out the top of the cabin. It got the job done. When I joined the Navy I found that the LC M, had three centrifugal bilge pumps two on the starboard engine and one on the port engine. They were fed from a small line that tapped off of the raw water cooling pumps ( they were impeller (Jabsco I think)) this kept them primed. Fast forward 20 years. I had a shrimp boat with what I was told was a 120 Ford diesel, anyway it was a huge stranger cooled with a 1 inch Jabsco pump belt driven at 50% of engine rpm. This pump was still in operation when I sold the boat six years later and I never changed the impeller. I guess it means a lot has to do with the installation. I believe the main reason for rubber and power pumps on most marine engines as they do not know where it’s going to be mounted weather above or below the water line. Just my two cents, Mike.

[Cheechako]

Originally Posted by emeraldsea

I skimmed through and don't see any mention of the fact the most pleasure boats send the exhaust out with the raw water, which acts as a muffler. Its crucial that the raw water pump turn only when the engine turns, as its the exhaust pressure that keeps the water from getting into the engine. Water in engine=(

This is a good point... and may be the best answer why centrifugals arent used..... In 40 years I have yet to buy a boat that had a vented loop in the raw water circuit. An impeller pump likely blocks the ingress of water when not running. A centrifugal has no mechanism to stop the flow of water!

[emeraldsea]

If the engine sits below the water line, and the raw water goes out with the exhaust, there has to be a vented loop. The water will slowly get past the impeller.

Another issue is rate of flow, which is engineered to be regulated by engine speed. Having too much flow when the engine is at idle I suspect could cause problems also.

I think having the ability to use a wash down or bilge pump to "get you home" is great so long as your aware of these potential problems.

[Cheechako]

Cant tell you why, but at least 3 of my boats have had this situation and no issues. Maybe due to not the whole engine below waterline and the exhaust mixer marginally above? not sure.

[psneeld]

Originally Posted by Cheechako

This is a good point... and may be the best answer why centrifugals arent used..... In 40 years I have yet to buy a boat that had a vented loop in the raw water circuit. An impeller pump likely blocks the ingress of water when not running. A centrifugal has no mechanism to stop the flow of water!

Doesn't block it enough or not guaranteed. Many genset maufacturers therefore DON'T recommend scoop thru-hull intakes for that reason on faster boats.

[Shanaly]

Originally Posted by David M

It comes down to impeller pumps being positive displacement meaning they are self-priming provided they do not have to pull against too much of a partial vacuum (pressure below one atmosphere) in order to get primed.

Centrifugal engine coolant pumps are low on the engine block which means the pump is already bathed in coolant keeping it primed.

.... so, if my impeller pump on my engine is BTWL, i assume i can replace it with a centrifugal car type pump. or even an electrical wash down one, altho i prefer a mechanical drive...

[sailorchic34]

Oh good a Pump discussion.... Love pump discussions

The rubber raw water pumps are used as they handle small debris easily and are easy to repair, compared to centrifugal pumps.

Centrifugal pumps would have excessive wear from the salt water and stuff in the water. Could you use one, properly designed for raw water... Yes. They have design issues with low suction head, but it's doable. They can handle variable speed just fine.

But it would wear out almost as fast as a rubber impeller and would require special tools to replace, (pullers, micrometers, etc) and are persnickety about clearances, etc. Not something you want to replace in a hurry on a lee shore

The rubber impeller design for lack of a better word is idiot proof, in that it is very easy to replace, with a screw driver and adjustable wrench. Even Blondes can do it.... It's not effected by low suction head can pass gritty water will little effect and is easy to repair..

Centrifugal work better in closed loop systems which is why the closed loop fresh water pump is centrifugal. It has issues with NPSH (low suction head) and the higher tip speed would cause more etching of the impeller and wear ring in salt water.

[Shanaly]

cor - something i can understand! makes sense, i shall henceforward pay the annual exorbitant impeller fee with a glad heart

[tartansail]

Apologies for the slight thread drift, but this didn't seem like it warranted its own thread and the problem is with my raw water system.

The nipple on my strainer is enough larger than the nipple on my raw water pump that a snug fit on the pump with 5/8" hose is all but impossible to get onto the strainer. Options: (1) Next larger hose (3/4") with Plumber's tape to beef up the nipple on the pump, (2) Enlarge the ID of the 5/8" hose with Dremel to fit the nipple or (3) replace the nipple on the strainer with a smaller size. I actually don't know that the last option is viable and is a relative PITA compared to the first two. So, is there anything to choose be Plumber's tape and growing the ID of the smaller hose? Is there a reason that both will create enough risk of leaking that I should bite the bullet and do #3?

[sailorchic34]

Me, I would change out the fitting on the strainer. I have a 1-1/2" dia. strainer connection, big monster, reduced down to 5/8"..

WM has male to male reducers too, but that adds a fitting and two hose clamps, well four hose clamps if done right in an area you want minimum fittings. Less fittings, less to go wrong....

[tartansail]

Originally Posted by sailorchic34

Me, I would change out the fitting on the strainer. I have a 1-1/2" dia. strainer connection, big monster, reduced down to 5/8"..

WM has male to male reducers too, but that adds a fitting and two hose clamps, well four hose clamps if done right in an area you want minimum fittings. Less fittings, less to go wrong....

Thanks. That's probably the best solution though I may take try one of the shortcuts first.